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Have crash investigators ever used pilot cameras to help find the cause of a GA crash?
With video cameras being completely ubiquitous and GoPro cameras able to be mounted pretty much anywhere, lots of GA pilots now like to make videos of their own flights. A passenger's cell phone video was used as supporting evidence in the Germanwings crash. Have there been any situations so far where a GA crash has been puzzling to investigators until they watch the video from the pilot's camera which shows the cause clearly? Are they even allowed to do this (anywhere, but NTSB in particular) or are there privacy regulations which prevent them from doing so?

@ymb1 Oh, now that's ironic!

I remember this case where a passenger camcorder was used to determine the speed of the engines, and the fact that the pitch of the propellers was reversed in flight in the landing phase, causing the accident. A spectral analysis was conducted on the tape soundtrack. The accident happened in a Twin Otter at Saint Barth, a spectacular approach (note the windsock horizontal position at La Tourmente pass).

Absolutely: Here's one of many examples, and you can search the NTSB database for "GoPro" and find plenty more.
The NTSB will use anything and everything available to them in their investigation, though release of certain materials is prohibited by law (e.g. CVR contents are not released, only transcripts).
I believe the NTSB is observing a similar protocol for onboard video: A synopsis is released (including a transcript of conversations) when requested, but the actual video is kept confidential by the agency -- I am not certain in that though, and the NTSB press office could provide a more detailed answer on their rules surrounding cockpit video / audio in GA crashes.

As a note, don't search for camera or video in the database: You will get thousands of results from "security camera" or similar which captured accidents & were referenced in the ensuing investigation.

The V Star appeared as a small, dull white flashing dot Do all V Stars flash? ;)

@TomMcW They do when they're equipped with a white flashing anti-collision light/strobe :)

I figured they were talking about a strobe. It's just the way they worded it makes it sound like the plane itself was flashing. The report didn't make a conclusion as to cause. Would that be the the Pitts pilot's fault since he was overtaking?


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What are those 5-character alphanumeric waypoints?
On the following High IFR chart from skyvector.com I notice along the meridian at 30' intervals there is a green symbol the same shape as an RNAV waypoint. Next to them are 5-digit identifiers, but it is alphanumeric rather than the usual 5-letter fix name. What are these symbols and what are they used for? Can they be entered into the FMS?

It is part of the hgh altitude redesign program.  You can read about it in the back of the Chart Supplements US booklet, formerly AF/D.

source: Stack Exchange

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Why was Air India flight 101 in 2018 cleared for the ILS at EWR?
On Sep. 11, 2018, Air India Flight 101 was flying from Delhi to JFK. On approach they had multiple failures including two of the radar altimeters and the localizer receiver. They decided to do an LNAV/VNAV approach and were having New York TRACON help them find a diversion airport that met the minima for such an approach. They ended up going to EWR. The approach controllers were aware of the failure of the LOC receiver and that they were unable to use the ILS. But they told them they were clearing them for the ILS to rwy 4R and that they were to fly the LNAV/VNAV instead.
The ceiling was reported at 400 ft. There are no LNAV/VNAV minima for 4R. The LNAV minima is 660 ft. 4L has a LNAV/VNAV minima of 470 feet. Was the ILS the only approach they could legally clear them for due to the reported ceiling? Was it legal for the pilots to fly it with only VNAV? I understand that the fuel situation was taking away their options  and they used the best option available. I’m just wondering why ATC decided to give them that particular clearance.
As a side question, why doesn’t 4R have LNAV/VNAV minima?

Were they legal to fly that approach "vis only", i.e. ceiling not required? (Could make it legal; doesn't make it necessarily a good idea.)

How could they be legally cleared for an approach that they were not equipped to fly?

Did they declare an emergency?

@Jim I don't believe so. I don't have a complete transcript of the ATC comms, just the videos on the internet.

The simple answer is that ATC cleared them because they could. The crew had advised a low fuel situation and no one in their right mind was going to tell them to play musical chairs with all the nearby airports having low ceilings. You also need to remember that when push comes to shove, the PIC can do pretty much anything they want to guarantee the safety of the a/c and pax in whatever way they feel is appropriate.

Once an emergency was declared -- if I understand it correctly, by ATC and not the pilots -- the options became broad. Most ILS approaches can be flown with lateral guidance only, but the minimums will be far higher. If the localizer is also missing, GPS will work for lateral guidance as well.
I'd expect the crew to have flown the approach with three sets of eyeballs, one on instruments, one on crossing altitudes, and the third looking for the runway environment allowing a descent to land.
The question I have is why they didn't go to PIT, as the ceiling there would have been a far safer bet than facing running on fumes after a possible go-round to EWR. Maybe the 7 ton fuel state was too dire for that. (Perhaps a 777 driver can help us here.)

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Why is the F-117A the only plane to use the fully faceted design?
One of the keys to the stealth capability was its faceted design. The concept is that curved surfaces have a larger radar cross-section. The idea of making all surfaces flat to reflect radar away from the receiver seems a pretty sound concept. It must have been pretty effective, since they had to treat the windows because the pilot's helmet had a major effect on cross-section.
But all of the low-observable aircraft made since, such as the B-2 Spirit and even Lockheed's own F-22 and F-35 have a significant amount of curved surface area.
Why has the fully-faceted concept not carried over into newer low-observable designs? Does it just have too much negative affect on handling characteristics?

The F-117 was designed at a critical point in aeronautical history where we didn't have enough computing power to analyze the radar cross section of more complicated geometry, but did have the computing power to provide artificial stability to unstable designs.
The faceted design is not ideal for stealth or aerodynamics. Obviously the corners are less aerodynamic than a smooth surface. But it also means that if the facets align with a radar source they will reflect a strong return. The compromise is that it's easy to calculate the reflecting angles of radar energy off of flat surfaces based on various locations of radar sources relative to the aircraft.
A curved surface means that only a small section of the surface is really reflecting directly back to a radar source, while the rest is scattered. Together with radar absorbing materials, this can provide effective stealth. It just takes more analysis to determine how different curves and features will reflect radar energy.
You can see the evolution of stealth designs from the Have Blue (prototype for the F-117) in the mid 70's, to the Tacit Blue prototype in the late 70's, to the B-2 in the early 80's.

This is an interesting concept regarding radar reflection, but did they really need powerful computers to tell them that a round object scatters radar better at all angles, vs a faceted design only being effective at certain angles?

The faceted design makes the aircraft aerodynamically unstable.  This is stated in the Wikipedia Article.  The designers had to use a complicated fly-by-wire system to compensate for it and is one of the first instances of fly-by-wire aircraft.  The F117A could not fly faster than the speed of sound because of this.  The F117A was designed to fly in places where there were surface to air missiles so they wouldn't be identified on radar and shot down.  It was not designed to participate in a dog fight.  Current stealth technology relies on radar absorbing materials so that the aircraft can be aerodynamically efficient.

Not to say that aerodynamics are not the major concern, but modern tactical aircraft are pretty much all aerodynamically unstable, to the point of being unflyable without FBW, including all of the low-observables. Despite its F- designation, the F117A is not a fighter. It was no more designed for dogfighting than the B-2, which is also not capable of supersonic flight.

"and is one of the first instances of fly-by-wire aircraft." The LLRV (1964) was one of the first FBW aircraft. Also the F8 (1972) used FBW. I kind of disagree that an aircraft introduced almost 20 years after the LLRV would be considered "one if the first instances". But I stand to be corrected.

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What is the reason for this unusual flight path?
Last week, flight AA9675 flew from MIA to TUL. It took this very unusual flight path:

It's an ad hoc flight number, so it's not a regularly scheduled flight. You can see there was a line of thunderstorms moving across at the time. But that alone does not explain the route. FlightAware shows the flight plan that was filed.

After leaving Miami it skipped the first few filed waypoints. But after it left Florida it followed the plan as filed from one VOR to the next, in a big circle around the Midwest and finally landing in Tulsa.
My only guess as to the reason for such an unusual route is that, due to the line of storms, it's some kind of contingency flight. Maybe they were anticipating delays and kept a plane airborne so that it could be quickly re-routed if necessary. I've been told that FedEx does this during peak season, but I've never heard of a passenger airline doing it.
Does anyone have an explanation for such an unusual flight path?

KTUL is American's engineering and maintenance center.  Probably headed there for a C or D check. Can't explain the specific route though.

They stayed over land, but there are plenty of shorter routes that would also stay over land all the time. Any chance they for some obscure reason had to go a certain minimum distance?

@Gerry Definitely in TUL for maintenance. On Oct 31, the same plane, N175AN made a test flight TUL/TUL.

Sometimes a minimum flight time is required to perform engineering checks. Examples could be if it is required to perform a certain number of tests which take time and which cannot be done on the ground, or if it is required to have the tested system operated for some time to generate the right temperature e.g. cold-soaking tanks etc. It could be one of these cases.

I wonder if this route isn't a tailwind flight test while in a depression at a good distance from the center. Was there a depression in this area at this time?

It probably was there for the weather, to give the best ride for pax.

Notice that they flew directly over (as best I could tell based on the image's resolution) MCO, ATL, SDF, IND, ORD, MSP, MSN, RAP, CYS, DEN, AMA, and OKC (in order).  Looks as if the waypoints were chosen to match a bunch of cities to which AA flies.  Perhaps testing the navigational systems of that A/C?

My guess is the captain requested and the dispatcher approved an excessive amount of fuel to deal with the crossing of the long line of weather.  The flight plan would have to reflect being able to burn off that excess fuel to be able to land BELOW the maximum landing weight of the plane. The crossing consumed far less fuel than expected so the crew had to burn off the excess fuel by flying the filed flight plan.  Or they could have just wanted to fly the filed flight plan because flight crews get paid by the hour and they would have just about doubled their pay for that leg.


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Why can't runway 18 at Frankfurt Airport accept takeoffs or landings in the opposite direction?
Runway 18 at Frankfurt Airport is one example of a runway which is numbered on only one side. Aircraft can only takeoff in the 18 direction, and cannot takeoff in the 36 direction, nor land in any direction except in an emergency situation.
What is the reason behind it?
Thomas Römer/OpenStreetMap data [CC BY-SA 2.0], via Wikimedia Commons

There are quite a few airports like this.  Another example is Heathrow where take-offs from 09L only take place in exceptional circumstances due to noise abatement.

Kind of like st.maarten airport.  You can only land and takeoff in the same direction.

Let us consider the operations other than takeoff on runway 18.

Takeoff Runway 36
There are buildings not far off the departure end and directly after the end of the runway is a movement area and ramp.  To safely takeoff they would have to clear that area and probably replace all the concrete with crushable concrete (more on this later).  The buildings probably post obstacle clearance issues for engine out climb performance.  The east-west runway north of 36 would have conflicting traffic and would have to be carefully sequenced.  Lastly, that purple area to the north of the runway is probably a noise sensitive area you aren't allowed to fly over at low altitude.
Landing Runway 18
You approach over a noise sensitive area, cross over operations on the northern east-west runway, overfly a building at extremely low levels (wheels on the roof) and have to make sure there are not airplanes on the ramp between the building and runway.  Landing short would be nothing short of disastrous and every landing would be a risky operation with the close proximity of airplanes and buildings.  Glideslope services would probably not be available due to ground obstacles.
Landing Runway 36
This is the least problematic approach.  The problem is in missed approaches and long landings.   There is no safety area at the end of 36 and a runway excursion past the end of the runway would be catastrophic.  To mitigate this they could re-pour the ramp concrete with crushable safety concrete, but that would probably make using ramp and building unsuitable.  If traffic goes around they conflict with aircraft on the northern east-west runway and have a noise sensitive area to deal with.

In comparison to the above, taking off to the south only requires a blast fence just north of the runway to protect the ramp and building from jet blast.  There are no logistic issues in aircraft movement on the ramp and the building is not in danger of being hit.

In short, you can take off with your back to an obstacle but the other three things are right out :-)

One more reason is the chemical plant nearly directly below the extended runway centerline north of the airport (very slightly east of it) in Höchst. I can imagine very well that this had an effect on the decision if planes should fly there or not.

looking at the chart, you can see buildings right past the treshold on the opposite end. Taking off to the north (or landing from the north) would put you dangerously low over those buildings, not a safe situation at all.
Also, there are no doubt very strict noise restrictions in place which ban overflights of the towns and city districts just north of the airport, further restricting the usefulness of maintaining navigation aids and other things needed to make it an operational runway in both directions.

If you see the runway orientation, it is clear that any landing in the runway 18 (Runway West) will interfere with air traffic in the other runways (Runways North, 07C/25C  and South, 07R/25L).
The same thing goes for takeoffs to the north in that runway.
This is the reason the runway is used only for takeoffs in southbound direction.

In a general case, that sort of an issue can be solved; Boston Logan (KBOS) and Philadelphia (KPHL) are good examples. They routinely land on runways that either intersect or whose finals or missed approach paths would cross.  To keep operations safe, they manage the spacing of inbound aircraft so that they're separated in time so that even if EVERY aircraft went around, they'd alternate crossing over the airport.  I suspect that the noise sensitivity is what drives the non-use of 36 takeoffs and 18 landings. (Not landing on 36... maybe the extra capacity just isn't that needed??)

Amsterdam has crossing runways that are operated in both directions, no problem with that. Just don't use them at the same time :) If your assertion were true the runway would not exist at all because the missed approach path crosses another runway.


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What is that building in Suvarnabhumi Airport?
In Suvarnabhumi Airport (Bangkok) there is a building in aerodrome, next to the main terminal (Circle by red colour) . What is that?

On page 5 of current airport chart states it is Transfer Baggage Terminal
It was officially opened on 1 November 2014 and reduce baggage transfer time from 75 minutes to 60 minutes. It was constructed above South Airside Tunnel between gate C, D and E and has usable space of about 12,550 sq.m.
Source (Sorry source is in Thai. My own translation.)
Another source in English. Similar to my original source.


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Do seaplane bases ever have marked "runways" or approach lighting?
Some seaplane bases have prescribed areas for landing, but are the landing areas marked by buoys or lights? If so are "runways" outlined in the water or just a general area?
Do they have IFR approach procedures?

Not reality but even after 50 years I still love Largo's gruff command in Thunderball: "switch on the underwater landing lights"

Are the landing areas marked by buoys or lights?
In at least one specific instance, Yes.
The seaplanes that operate out of Lake Union, Seattle, have "Advisory Buoys" that help make the many sailboats, kayaks, and motorboats on the lake aware that planes will be arriving or taking off.
The buoys have Pilot Controlled Lighting (PCL) that will turn on yellow flashing lights.  They are typically activated approximately 3 minutes before any departure or arrival.
The buoys do not restrict boat movement; they are advisory only.

I am not aware of any IFR procedures for a seaplane base, but I do not know for sure.

The busiest water aerodrome that I know of is Vancouver Harbour (CYHC) in British Columbia, Canada, which has over 50,000 movements a year, as a hub for a large network of seaplane commuter routes up the coast and across to Vancouver Island and down to Washington State.  Busy enough that it's also the only water aerodrome that I know of that has a positive control zone with a control tower (at the top of a harbourfront office building).
There is no instrument approach however, and airplanes land in a general area in front of downtown as necessary for winds and to avoid boats, riptides, deadheads, and the wakes of ships.  All the flying in and out is VFR, although often in conditions you would think of as IFR.

There are a number of controlled airports with water runways, though there may not be any others with just water runways and a control tower. For example, Honolulu, HI is a Class B with two water runways 04W-22W and 08W-26W and there are two VOR approaches to them.


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What causes the adverse pressure gradient that leads to flow separation?
When reading about flow separation I generally see a diagram like this:

imp source: Wikimedia commons
This shows that the boundary layer slows as it passes over the wing, eventually creating an adverse pressure gradient. The effect being more pronounced close to the wing it will cause a reversal of flow and separation of the boundary layer.
I can easily see how this leads to flow separation, but I’m unsure as to what causes the boundary layer to slow down like this. I’ve read that the effect is independent of surface friction.
I know the air at the surface is stationary wrt the wing and its velocity increases the further away it gets. But if I look at the arrows in the diagram and select, say, the second one from the surface, it slows down as it goes along the wing, eventually reversing. At first I thought that the still air at the surface was somehow affecting it, but I don’t know how (shear stress?). But even at that, stationary air can not make it flow backwards.
So, what causes the flow closer to the surface to slow?

Friction plus pressure rise.
No, the effect certainly isn't independent of surface friction, otherwise the location and angle of attack of separation wouldn't change with Reynolds number.
First, friction will prevent the surface layer to become as fast as the outer flow in the suction peak, and once pressure rises, all air will slow down by the same amount. Now the already slower part near the wall will actually reverse because it slows down to a standstill and is attracted by the low pressure ahead.
Pressure has to rise past the suction peak in order for the air to get back to ambient pressure. The upper side suction is caused by the airfoil's curvature, and curvature  over the rear part of the airfoil is very low or even negative - that is what makes the air assume ambient pressure again.
I think you know already my slightly longer answer on that topic, but I will link to it nevertheless. Please let me know if I need to explain more!

‘and once pressure rises, all air will slow down by the same amount.‘ What causes the pressure to rise after the suction peak? I get into the Bernoulliian conundrum of “pressure rises because the air slows. The air slows because pressure rises.” I don’t quite get what’s causing it to rise/slow past the suction peak.

It's the geometry! A concave curvature causes an increase in pressure. To that you have to add the tendency of the air to return to atmospheric pressure (in subsonic flow). So even with a slightly convex surface, you get a gradual increase in pressure. Just like the air flow speeds up at the nose because there the curvature is strongly convex (small radius of curvature) at the nose of an airfoil, so it creates a suction peak.

@TomMcW: Thank you for asking; I added another paragraph. But also note Daniel's comment, which is spot on.

@TomMcW For the simple reason that the zone of suction - the suction peak - has a beginning and an end.

@PeterKämpf Why would an adverse pressure gradient form in the case of a cylinder, where surface curvature doesn't change like an airfoil does? Why wouldn't flow keep accelerating toward the rear part of the cylinder?

@Frank acceleration needs energy- where should that come from? Bernoulli says that the sum of kinetic energy (speed squared) and potential energy (pressure) is constant. Regarding the cylinder: Its curvature causes a strong suction peak at the top and bottom, so pressure has to be brought back to ambient somehow. Since the remaining flow path is too steep, separation creates a wedge such that the outer flow can recover pressure gradually.

@Frank Please ask a new question instead of lecturing about potential flow.

@PeterKämpf Sorry, please let me ask another question: Will the slower flow near the surface (due to friction) create a less intense pressure gradient? Why will all air slow down by the same amount?

@Frank Air slows down differently in the boundary layer. I have the impression that you have major misconceptions about aerodynamics which are hard to pin down from brief comments. Please ask a proper question and use it to explain your view. I will not answer more questions in comments.

@PeterKämpf Thanks for pointing that out. I asked a new question on this topic: https://aviation.stackexchange.com/questions/100351/why-will-all-air-slow-down-by-the-same-amount-in-an-adverse-pressure-gradient Hopefully that explains my view.


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What is a lambda wing?
I was reading a description of the Boeing X-45 that described it as having "lambda wings." What is a lambda wing and why is it called that?

Indeed, it is due to resemblance with Greek letter 'lambda', though not the capital one ($ \Lambda $), but the small one: $ \lambda $.
I am attaching a pic of X-45 with me drawing a lambda over it. Typically, the wing has two taper ratios. The inboard wing tapers heavily, corresponding to legs of small lambda, and the outboard wing tapers slightly or do not taper at all.

Other examples include B-2 bomber:

This answer seems to be correct.  Related link that could be included in answer : https://fas.org/man/dod-101/sys/ac/docs/fatereport/sld034.htm .  Google "lambda wing" for more.  However it's not impossible that at one time in the past, the term would have been applied to the wing of the English Electric Lightning, etc.

I must be missing something. You stated, "a pic of X-45 with me drawing a lambda over it". Unless it's a GIF that's supposed to actively show you drawing a lambda and whatever mechanism that's supposed to show GIFs is disabled in my browser, I'm not seeing anything drawn over the X-45A.

@FreeMan - notice the odd black mark on one side?

Wow. No, I completely missed that. Hangs head in shame

It's ok!  Really...

It's because the shape of the wing resembles the uppercase Greek letter lambda with the leading and trailing edges having the same sweep angle.

What makes it different from simply “swept wings” though?

Here's a presentation from Lockheed Martin:  https://fas.org/man/dod-101/sys/ac/docs/fatereport/sld035.htm

I see—the confusing point is that there are actually two very different X-45 concepts. The X-45A has swept wing and fuselage with pronounced chines, while the X-45B/C is a flying wing. According to the presentation, the term applies to the later.

I had read through that presentation before I posted the question, but it isn’t at all clear what distinguishes a “lambda” planform from any other swept wing.

I’m no aeronautical designer, just a dude who knows why that particular wing shape is called ‘lambda’. An educated guess is that a ‘swept wing’ is a more generic term for wings of a configuration with a swept leading and trailing edges unlike a Piper Cub, for example, which has straight leading and trailing edge. There may be several distinct ‘swept wing’ configurations with curved leading and/or trailing edges, but the ‘lambda’ configuration is unique, with straight leading and trailing edges. Again, I’m no expert, and would like to know if there is any deeper explanation.


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How much N1 difference between two engines are acceptable?
While I was flying with 10+ year Airbus 320, IAE engine with EPR, I saw a constant difference between N1 parameters around 2%. I checked the Engine Hours and Engine Cycles that they are almost the same. Oil consumption of a greater N1 valued engine was less than the other one.
So why N1 parameters different? is it a sign of an engine part problem?
Note: 2% value difference at cruise level, on the ground it is around 1%

Can you match the N1s by splitting the thrust levers?  And what are the N2 or EPR values?  Are they equal while the N1s are different?

Good question that could be extended to "how much thrust, temperature, N2, EPR differences are acceptable for a 2 jet engine aircraft?"

What does your mechanic says? Sure airline like yours will have maintenance guys on hand to answer this.

@JohnK EPR values were the same N2 differences were less than 1%. Generally, I did observe N2 differences for older engines less than 1% but no difference at N1 values. If it is helpful around 20 degrees Celsius difference at EGTs from take-off till landing.

@vasin1987 I asked the to our base mechanic about the difference they said they do also monitored the difference from engine performance logs but they could not give a clear answer due to the engines are at the same cycle and hours which I suspected in the first place.


text:
How much fuel does it take to start/land an aircraft, compared to normal flight?
I'm wondering how bad it is to fly a short distance, such as ~50km, with a big airplane. How much less fuel does it take to fly at high altitudes compared to low? And how much of the fuel is spent on start/landing? Flying between continents is never a problem, but if a politician flies a shorter distance all hell breaks loose.

landing is with engines mostly at idle (you are descending and slowing down most of the way down).

The hell breaks loose not just because short flights are more expensive per mile than long ones but mainly because due to the time spent on the airport and need to drive to the airport and get cars at the destination airport to drive from there the time saving compared to just going by car is small while the cost is significantly higher. And it's not just fuel cost, but also some maintenance depends on pressurization cycles, i.e. flights, and there is all the more complicated organization that also costs something (especially if we talk about government plane, not regular airline).

In this case, a swedish politician flew 41km to another airport, where a few other politicians were picked up before they continued to new york. So it's a matter of two cars going 50km and back (200km total) vs the government airplane making the short flight to pick them up.

The fuel consumption or fuel burn rate is dependent on the aircraft type, size, engine configuration, cruise altitude, temperature, thrust setting and other factors.
In general: The larger the aircraft, the less economic do short flights become due to weight/size and fuel burn on takeoff/climb.
Here are some values I found for the CRJ2, the source may not be entire citable:

Approximate total burn pounds / hour
Taxi: 700 (Both engines and APU)
Takeoff: 6000-7000 (depending on temperature, elevation, reduced/full thrust, etc)
Climb: 6000-3500 (reducing as as altitude increases)
Cruise: 2500-3000 (depending on altitude/weight/etc... subtract 500 for long range, add 500 for high speed)
  Holding: 1800-2400 (depending on altitude/speed)
  Normal descent: 1500
  Idle descent: 600
  Approach: 2000 (fully configured)

(Source: www.airliners.net - Author: Flyf15)
You can also read the following related questions to understand how altitude affects fuel efficiency:
Why do jet engines get better fuel efficiency at high altitudes?
How does flight duration affect the hourly fuel consumption in heavy aircraft?

The uproar was justified. The Swedish politicians should have driven to one airport and flown from there in one leg.
There are two effects which make short flights inefficient:

The relative length of unproductive taxiing and initial acceleration becomes bigger the shorter the flight is, and
Flying at low altitude is slower, so much more fuel is consumed per distance flown.

You also need to burn more fuel for climbing, but you will get that back on descent when the aircraft can reduce thrust, just like you can coast downhill in a car.
Jet aircraft reach their highest efficiency when flown near the tropopause. Here, air density is only a quarter of what it is at the ground, and the aircraft will fly twice as fast up there when flying at its optimum lift to drag ratio. In addition, the colder air at altitude will make jet engines more efficient. Therefore, flying those 41 km used more than twice as much fuel per km traveled than flying from Sweden to New York. Add to this that the aircraft needed to taxi for several km twice instead of only once, and also needed to spend the fuel for the take-off run twice. While the potential energy added during climb can be regained during descent, the initial acceleration energy is mostly wasted as brake heat, spoiler turbulence and reverse thrust. This adds up to several 100 kgs of fuel for a large airliner.


text:
Is there something like an ignition key for planes?
Is there something like an ignition key for planes?
Or could one "just enter" and fly away if they could?

The planes I have trained in (single engined Piper Cherokees/Warriors) do indeed have an ignition key, without which the magneto system (which drives the spark plugs) and the starter cannot be engaged. Without being certain, I would assume the same goes for other small GA aircraft.
When you get to airliners I have no idea whether there are keys for locking the controls - as the controls are much more complex, the keys probably won't be directly connected to ignition systems or starters, and I wouldn't be surprised if the primary security method is keeping doors locked.

That being said, in small planes it's much easier to hot-wire an airplane than even a car.   All the keys do is to ground the p-lead in each magneto preventing it from generating spark. And when you turn it it disconnects the ground and engages the starter.
So if you don't have the keys, it's simple.  Open th. Close cowl.   Then starting it is as simple as an old school hand prop start.   Someone in the plane holding the brakes and controlling the mixture, throttle while (hopefully not you) is outside and hand prop the plane.

Pretty sure I remember seeing on a documentary that a key was involved in the hand-over ceremony as a new (large) airliner was delivered from the manufacturer to the customer (airline). However, the key may have had more of a symbolic purpose than a functional one.


text:
How are attitude indicators kept accurate?
The attitude indicator is very important for any sort of instrument flying, so it must be accurate at all times regardless of the plane's movements.
I presume that the indicator would be connnected to a gyroscope. However, gyroscopes drift, and a drifting attitude indicator would be pretty useless. Of course, you might have a reset button, but safely using it would require a visual reference to a horizon, which obviously you don't have during IFR flight.
Another idea might be to periodically reset the indicator based on gravity --- many model aircraft use this something like this approach to drive an attitude calculation; however, during coordinated turns, the gravity vector is indistinguishable from the vector during level flight. If a plane continually turns in a coordinated way, such an attitude indicator would be confused in exactly the same way that a human would be, resulting in bad things like the autopilot doing a graveyard spiral. A gravity sensor + accelerometer wouldn't be able to detect the difference between a gradual turn into a steeper and steeper coordinated spiral and simply a gradual gyroscope drift while remaining in level flight.
How are attitude indicators kept from drifting?

gyro drifts are corrected by external sensors, such as GPS. Could you rephrase your question to focus on one of the following: 1) how are (mechanical) gyros of planes reliable after long flights? long years of use? 2) how can mems gyros / IMUs be kept reliable?

How can GPS correct the gyro of the attitude indicator? I'm not talking about INSs here, and I think this is very obvious.

Do you want to know about gyroscopic AI, or EFIS AI? Then do you want to know about preventing errors, or correcting (resetting) errors in flight?

If I am not mistaken, an EFIS still uses a gyroscope, albeit connected to a computer instead of directly to a ball. My question is more a theoretical one: how can an AI be accurate if no sort of inertial measurement seems to be able to distinguish between a long coordinated turn and level flight? And in IFR flight, there are no outside cues that can be used to reset errors in flight (imagine 8 hour international IFR flight at night)

The answers are all in this question: If I fly a balanced turn long enough, will the AI show I'm level?

A gyroscopic AI has an erection mechanism, which continuously corrects the AI to be upright based on the local level, or downward acceleration vector. The correction rate is generally 3-5 degrees per minute.
The way that the AI corrects itself is a system of pendulous vanes. When the gyro is not upright relative to the local level, centrifugal force pushes open vanes on the gyro's case. Air escapes through the uncovered holes, applying a force to the case, and correcting for precession.

(FAA AC 65-15A)
The second half is somewhat counter-intuitive, but flying in a coordinated turn will not continuously increase the error in the AI. For the first half of the turn, the AI will add error; in the second half, it removes the error. After a 360 degree turn, the precession error will be removed completely. For more detail, see another answer: If I fly a balanced turn long enough, will the AI show I'm level?

I'm not confused over the correction of precession error. However, let's say somebody stays in a coordinated turn for an hour. Then the erection mechanism (which I do know about) would be confused, right? It would "erect" to a slanted position because in coordinated turns, the acceleration vector always points down? i.e. the gyro is always "upright relative to the local level".

That's the counter-intuitive part: the erection mechanism does get "confused" by the different down-vector, but averages out across the entire circle. Think of it this way: in a 360 degree circle, the "down" vector sweeps out an entire arc - so on average, "down acceleration" is in fact straight down. (Not a perfect metaphor, but should help your mental picture.)

The simplest answer is that the error introduced by gyroscopic precession is not considered significant during normal aircraft operation (maintaining relatively normal flight attitudes, i.e. within 30* of level flight). There is a mechanism designed to automatically re-orient the gyroscope during flight, called a "caging mechanism", which normally works very well but during sustained maneuvers it can actually exacerbate precession error. The caging mechanism can usually be activated manually in VMC (or at least when the horizon is visible) with a "push to cage" button to ensure the AI matches the real horizon.
Theoretically, the error induced by precession over time could add up to be significant. Violent maneuvers increase the effects of friction and precession in the gimbal system and can cause it to lock or tumble. These situations are unlikely to happen undetected by the pilot, for several reasons:

Pilots flying by instruments don't use just one of them; there is some overlap in what the instruments should be telling you. For instance, at an indicated "cruising" airspeed where AOA is low, if the AH indicates level flight but the VVI or altimeter say you're sinking, or the TC indicates bank or any of your course indicators (DG, CDI/OBI etc) indicate a change in heading not mirrored by the slip indicator, that casts doubt on the accuracy of the horizon.
During IFR flight, the pilot is required to coordinate with ATC. In most cases, ATC can detect on radar that a plane is deviating from course, and will advise the pilot of this and coordinate with him to resolve the problem, including diagnosing any instrument failures that may have led to the deviation in the first place.
Pilots, no matter how experienced with instruments, don't like flying in IMC. It's just no fun; it requires a high level of concentration and a basic departure from the "trust your senses" mentality that has been the survival mantra for the human race for millenia. The potential for pilot error is high. Pilots will therefore seek to minimize their time in IMC by working with ATC to climb above or navigate around regions of IMC.
Modern instruments incorporate solid-state accelerometers and magnetometers as well as gyroscopes as the basis for the inertial navigation system, especially for the AHRS subsystem of glass cockpits. Gyroscopes may still be present in the system but they can be much more easily detected to be out of calibration and corrected by the computer. The horizon indicator included in modern "primary flight displays", therefore, is pretty trustworthy.

I've had this same question and even asked my flight instructor. He said the attitude indicator does not drift because it is on a 2-axis gimbal and rotates freely and will not precess or become incorrect unless it tumbles (reaches the physical limits of its movement), such as happens when doing aerobatics. A directional gyro, however, will drift due to the fact that it is only on a 1-axis gimbal and is deflected against when the aircraft accelerates, turns, or maneuvers. I'm still not sure how it is not subject to precession as it is a mechanical system and thus subject to friction and forces, but that's what the CFI said. He looked puzzled by my question (the same one you are asking) and answered as if the attitude indicator is never wrong.

Welcome to the Aviation Stack Exchange! Unfortunately, your CFI is confused - the AI definitely does need correction for precession. The description you're giving is a decent comparison between a DG and a wet compass, but even a perfectly frictionless gyro needs to be corrected for apparent precession.

Do you have a technical description of how this works/how to deal with this in flight? How long does it take in normal flight for such errors to manifest themselves?

In straight & level flight, the errors don't manifest themselves because the AI is constantly correcting itself. See my answer (and the answer linked to it).


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Are birds aerodynamically stable?
This question is bordering between aviation and biology. I am posting it here, since it is really about aerodynamics.
At first, one would think, that birds should be aerodynamically stable to save themselves the effort of maintaining stable flight.
However, even if humans had rigid joints they are in an unstable configuration other than four-legged animals. Thus, stability does not seem to be a must for biology, at least in certain aspects.
So, are birds aerodynamically stable? Are all of them, or are some species which are and some which aren't?

They are not approximable with a rigid body, hence *dynamic stability is not really an applicable concept. You could ask about a non-flapping, wing-extended configuration, but even then I might have some reservations.

@Federico why does it have to be (approximately) a rigid body to be aerodynamically stable? I think you could argue if a bird can glide with all muscles relaxed, you could call it aerodynamically stable.

@Federico similarly, you could also ask if there is a combination of joint angles in birds that would make them aerodynamically stable if fixed, thus removing the need for constant adjustments to remain in control.

@falstro you are making the bird a rigid body. Rigid does not mean "muscles contracted", means "joint fixed in a certain position". Obtain the joint fixture with relaxed, contracted, or any combination of muscles, and you can approximate the bird with a rigid body.

@Federico in the second case yes, the first case I'm talking about muscles relaxed, wings/body parts free to move about (within limits obviously, joints generally can't bend all the way around...). I'm no expert, but it doesn't seem implausible to me that a bird can be constructed in such a way that if perturbed from level flight the wings move in such a way as to return it to level flight, thus being aerodynamically stable.

Have you done a basic literature search first? I think a lot of control theory & mechanics knowledge will be needed to get anywhere in this subject... (Also, I think it's beyond aviation too but I'll let others decide.)

An example paper here - has a basic analysis of the sort I was thinking of. (Needs control & root locus and all that stuff.)

Hmm... The on-topic page says that aerodynamics questions related to aircraft are on-topic here. Since a bird does not normally qualify as an aircraft, I'm inclined to agree with the off-topic close votes. Also, I agree with Federico in that you'd need to specify a particular bird and a particular configuration for this to be answerable regardless of what site it's on. You're probably more likely to find bird experts at [biology.se] and the question should be on-topic there, too.

I don't agree with the idea that this question is off-topic or more related to biology. Laws of aerodynamics apply equally to aircraft and birds. The question may be impossible to answer because this tight coupling of the aerodynamics and fly-by-wire control system in birds, but the same principles apply.

Location and semantics aside, is the answer not terribly obvious for a vast majority of bird species?

In reference to the current verbiage and the final question: is a hummingbird stable wrt flight?

Birds have integrated active control systems that rely on several sensors to adjust the control surfaces to stabilize the flight. Since it is very difficult (perhaps impossible) to study the aerodynamic properties of the bird independently from its control system it is hard to make any claims on the static or dynamic stability of a bird.
What can be said is that from a control system point of view, most birds are observable, controllable and stabilizable in flight. This does not apply to the class of flightless birds.
If you are interested in the similarities in aerodynamics between insects, birds and aircraft I recommend getting a copy of "The Simple Science of Flight; from insects to jumbo jets" by Henk Tennekes, an  Emeritus Professor of Aerospace Engineering at Pennsylvania State University.

After sitting in the garden on a sunny afternoon, observing various bird species and tinkering a bit more on the subject I actually think that many birds are unstable. Especially the smaller species that fly in the garden through bushes and trees, but also those that hunt for flying insects, are capable of extreme rapid changes in direction of flight; they are very agile.
Stability and extreme manoeuvrability are difficult to combine. In fact modern jet fights are designed to be unstable to improve their manoeuvrability. They then need a computerized control system to stabilize them.
An online search showed that some research has been done on the field of flight stability of birds Petter Krus of the Linköping University in Sweden has published an article on Natural methods for Flight Stability in Birds.
Abstract:

There is a stark contrast between the behaviour of flying man-made
  artifacts and the effortless elegance of bird flight. In this paper,
  mechanisms are proposed that seems to be at least related to the way
  birds fly when they are gliding (the no propulsion case). It is
  recognized that biological structures and actuator systems (muscles)
  are characterized by a great deal of compliance that produce a
  completely different dynamic behaviour of flight. In this paper it is
  demonstrated how this can be applied on a geometrically unstable
  bird-like configuration. The result is that geometrically unstable
  configurations can be stabilised without the need of rate gyros and/or
  accelerometers. Although the bird might have a fast acting control
  system for flight control, it is unlikely that it is fast enough for
  active stabilisation of the short period oscillation, and this is
  unnecessary with the model described here, which only requires
  stabilisation of the long period oscillation.

In his introduction he also briefly refers to the work of Tennekes, which does not include much about the stability of birds. Also in the introduction he observes that birds have a cambered wing and must have an aft centre of gravity. This combination makes the bird unstable (if it were a rigid body).

Hi DeltaLima, your garden observation put you on the right path. Birds are indeed unstable, in pitch, yaw and roll. The amount of instability depends on the wing planform and tail incidence, but in no wing position is a bird stable in yaw.

For gliding, it should be possible to build a replica of the bird and study "the aerodynamic properties of the bird independently from its control system". I wonder if anybody tried.


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Why are military drones shaped so strangely?
Military drones, such as the Predator drone or the Global Hawk, tend to have a weird shape with a whale-like head, and engines concentrated at the back. The wings tend to be of very high aspect ratio, and v-shaped tails seem to be popular:

Why aren't there, say, Cessna-shaped drones? If Cessna 172s work fine for human pilots, why not for computer pilots? The Cessna 172's low speed and stability also seems like a good asset for military recon drones.

Don't read too much into it, the designers are just overcompensating for something...

I'd guess that a pusher configuration helps in reconnaissance: there is no engine or propeller blades obstructing the field of view of the sensors and cameras.

Are military drones shaped differently from civilian drones?

You're assuming that a Cessna is the natural shape for a plane, but a lot of the design of a light aircraft is predicated on the thing having to carry one or more human beings, who need to see, be protected and so on.

The whale-shaped forward fuselage covers a parabolic antenna for a high-bandwidth datalink. The operators want to receive the reconnaissance data in real time, and by giving the antenna the best place in the aircraft, it will be able to connect to communications satellites even when they are just above the horizon.

Global Hawk cut-away drawing (picture source)
The V-tail of the Predator is an inheritance from its origins as a Navy drone which could be folded to fit into a torpedo tube. Read all about it in this answer. If you want more info on the benefits of an inverted V-tail, make sure you read KeithS's answer, too.
In the Global Hawk the V-tail was chosen to mount the engine above the fuselage for better IR shielding from below. The two tail surfaces also shield the exhaust from the side without affecting aerodynamics negatively.
The Aerosonde's configuration was chosen after Tad McGeer had experienced in 1990 from the 1/5th scale RC model of the Perseus UAV of Aurora that a tail-mounted propeller would stabilize the aircraft to the point that it became practically uncontrollable. A pusher propeller is more efficient than a tractor propeller, which also explains the propeller location on the Predator.
If it strikes you how many of the UAVs have V-tails: This allows to use fewer components (surfaces, connectors, actuators), and since all are moved by computer control, the issue of a mechanical mixer does not exist. The lower damping of V-tails can also be easily tolerated, since the FCS can react much more quickly and commensurately to disturbances than any human pilot. The lower maneuverability possible with V-tails is also not an issue - reconnaissance UAVs fly mostly straight and level.
The high aspect ratio wings are helping in extending the range and especially the flying time of observation drones. Similar to glider wings, the long and slender wing will offer the lowest drag for a given lift at subsonic speed.
In the end, each detail has been chosen deliberately and can be well explained. There is no conspiracy to make UAVs look different from Cessnas.

There is no conspiracy to make UAVs look different from Cessnas. Whew!  :)

Look at the F22 or the F35 they both have V shaped rear surfaces too.

@JamesRyan: In their case in order to reflect radar away from the horizontal plane. Both Global Hawk and Predator are not shaped for stealth, so radar reflection characteristics were not the reason for the V tails.

@JamesRyan, PeterKämpf - It might be more complicated than both of your stances. The angle of the "vertical" stabilizers on the F/A-18 is to get them out of the direct path of leading edge vorticies (a problem for the F-14 and F15). It also appears that some designs use inward deflection of both rudder surfaces to add to upward pitch control. And yes, avoiding right angles and aligning edges contributes to reduced radar return. As to what the primary reason(s) is(are), only the designers could say for sure, and some details of design and motivation are likely classified.

Please you add a parenthesis t explain what you mean by "The Aerosonde configuration". Did you mean "The Aerosonde's configuration" (so the subject is the Aerosonde in particular), or did you mean somethng generic about the Aerosonde (e.g. placing the pusher behind he wing but in front of the tail)?

@ChrisW: I meant that particular implementation of this configuration (which was used earlier in other designs). I hope adding the genitive-s improves the answer.

Yes that's unambiguous now, thank you. The illustration in the answer shows a jet engine behind the wings and in front of the tail, without the genitive-s it wasn't clear to me whether perhaps that's what you meant -- something generic about the configuration, generalised from the Aerosonde as an early prototype.

@ymb1: No, the prop created considerable lift and side forces, depending on angle of sideslip and attack. Since it was sized for flight at 24 km, it was huge and acted like another fixed tail. More on that here.

The biggest reason UAVs look like they do is because they don't need to carry a human, or the life support and avionics displays that human would need. The designs, therefore, are built around what the aircraft does need, mainly communications and viewing/recon equipment in addition to the standard stuff any combat aircraft needs (fuel, weapons, propulsion, lift/control surfaces). The use of a V-tail in practically all of them is for drag reduction; the aircraft doesn't have to be as maneuverable as a manned fighter, so a twin tail with separate all-moving elevators is superfluous, and the lower drag increases flight time for more range or loitering capability.
A key design consideration in any aircraft is weight balance. The 172, a fairly conventional aircraft design, is designed for a high degree of flexibility in its payload and distribution; the high wing keeps the center of gravity below the center of lift for stability, and by the same token the cockpit and cargo area are under the wing so changes in CG don't cause quite as drastic a change in handling.
In an unmanned aircraft, weight distribution is fairly static; any ordnance is right under the wings and thus the center of lift (fuel is also placed similarly), while the contents of the fuselage besides fuel load hardly change at all between flights. That allows the aircraft designer to allow form to follow function; the front of the aircraft has most of the avionics and communications, counterbalancing the propulsion system at the rear, with the fuel in the middle. The relative size and shape of these three elements is at the designer's discretion, and then the wings are simply placed at or slightly behind the balance point.
The RQ-1/MQ-1 Predator is probably the weirdest-looking, because in addition to having a V-tail for the weight and drag reasons, the tail is inverted:

The inverted V-tail has several advantages in a plane the Predator's size and with the complexity of the total flight system:

A rudder surface placed beneath the CG, as on the Predator, will roll the plane toward the direction of yaw instead of away from it as a top-mounted rudder would, which allows for coordinated turns using only the rudder.
This "rolling rudder" behavior is also pretty much all the RQ-1/MQ-1 needs in terms of roll control, so the rudder surfaces can be the only control surfaces on the plane, dramatically simplifying the control layout of the craft. Theoretically the lack of ailerons allows the wings to be removed or folded easily, but the MQ-1 has weapons pylons with firing circuits that complicate this. Larger drones with longer wings have higher MOI in the roll axis, more than the rudders could overcome by themselves, so these designs have ailerons (either wing-warping or hinged), which allows the V-tail to be upright for better ground clearance on landing.
A crosswind on an inverted V-tail will cause the plane to pitch up and roll into the wind instead of pitching down and rolling away from it with an upright V-tail. Given the high comms latency between Nevada and Kandahar, this is an advantage, as the plane usually has plenty of room above it to fly while the pilot notices and reacts to the disturbance, but can only fly downward for so long before it runs out of air. Again, larger planes have higher MOIs and so disturbances are more minor.
The inverted tail masks a little of the plane's engine noise in flight from specific forward angles. Not much, but every bit helps when you're trying to stay hidden.
Lastly, the inverted tail protects the more expensive propeller if the pilot scrapes the back end on the ground. The larger Reaper has more ground clearance to allow for the weapons pylons, and so it can flare more on landing without involving the prop.

The RQ-2 Global Hawk actually doesn't look that farfetched; it bears some resemblance to the A-10 Warthog:

The two aircraft have vastly different mission profiles (the Global Hawk is intended as the U-2's replacement for high-altitude surveillance and reconnaissance while the A-10 is a venerable armor-killer close air support craft), but there's some common design elements, such as the high engine masked by the tail surfaces to reduce IR signature, and the big low wings producing a lot of lift (for a high ceiling in the GH's case, for payload and survivability in the A-10's).
What's not similar between the two is program costs; the original A-10As had a unit cost of just \$450k in the '70s, and with new wings and a glass cockpit upgrade the A-10Cs are still just \$11 million a frame. Unit costs of the Global Hawk, including R&D, make the program the most expensive small aircraft to date at \$222 million per unit, eclipsing the "cost is no object" F-22 (\$182m).
Other drones are much more cost-efficient; the Predator is just \$4 million each while the larger and more sophisticated Reaper is \$16 million. Losing one still hurts, and the USAF has lost plenty, mainly to operator error (the communications latency inherent in piloting one of these remotely from a ground station half a world away is considerable), but the total costs of these programs is still a pittance compared to any manned airframe in service today, with dramatically reduced maintenance to flight hour ratios and total costs per flight hour. The A-10 is again the cheapest manned combat aircraft in service, costing about \$18,000 per flight hour to operate (no word on what all that price includes; likely some combination of fuel, pilot pay, parts, maintenance and ordnance). The Predator costs just \$3,600 per flight hour, and the Reaper about \$4,800. Again, the Global Hawk program is fairly expensive to operate (about \$49,000 a flight hour) and that's a big reason the U-2 Dragon Ladies that Global Hawk was supposed to replace are still flying (the U-2 is only about \$30,000 a flight hour).

I believe some other answer here mentions that the take-offs and landings are actually controlled by local operator on the airport exactly to avoid problems with latency.

How much work has been done on automated systems to allow UAV self-recovery/stabilization in those very cases where the operator connection is lost? It seems like something that would be worth the investment, or does that already exist and the main issue is operator control being lost during maneuvers that the UAV cannot self-recover from in time?

To elaborate on "don't need to carry a human," a big difference in the "weird" look (if not the shape) is the lack of a windshield for humans to see out through. I suppose that's obvious but it hadn't been explicitly stated yet. To me it makes the drone feel kind of "faceless" (or eyeless).

@Jan Hudec - I mentioned that in a previous revision of this answer, but that bit is less about looks which was the crux of the question. Yes, takeoffs and landings are handled by a local ground station in Kandahar while the bulk of the mission is flown from some cargo crates in Nevada.

@LarsH As far as lack of cockpit, the "cockpit" is basically the bubble cam you see on the bottom of the Predator pic (there's one under the "chin" of the GH too, but that plane is deceptively big, another consequence of having no cockpit is that there's no sense of scale to these aircraft in flight).

@JAB - The ability to tell the drone "go here" in very non-flying terms is actually a big part of the Global Hawk's capabilities (and its cost). The plane is normally controlled by just clicking on a map to draw waypoints, and the GH will figure out how to get where it's told to go, and can follow the flight path in the absence of further input. We used to do the same thing with cruise missiles, but a retrievable aircraft is less expensive to operate (though the breakeven on the GH program over a Tomahawk-based recon platform is on the order of 300 missions for each of the 50 GH aircraft).

The downsides of this control scheme so far are that the GH isn't smart enough to avoid weather, and doesn't have weather radar or even de-icing equipment, so the mission is scrubbed if bad weather's even likely (over the Pacific, that's close to a guarantee, leading to the Navy's version the MQ-4C which is more hardy)

@KeithS Oh, that's cool. But can it handle sharing airspace?

That's another big downside; the GH can't interface with civilian ATC by itself, so when one is being launched or retrieved from Beale AFB near Sacramento and the Bay Area (or anytime it's cruising in FAA airspace), either the plane has to be flown "manually" from a ground station patched into ATC, or NOTAMs issued and ATC instructed to vector civilian flights around the GH.

To forestall further discussion in comments, here's a really good article on the RQ-4 Global Hawk, what it can do, what it can't, what it costs, what is may save, etc: http://foxtrotalpha.jalopnik.com/why-the-usafs-massive-10-billion-global-hawk-uav-was-w-1629932000

Directly piloting UAVs is just a blatant make-work job. Human involvement should be limited to a high level "go to X, do Y". The computer knows better and easily monitor every sensor on the airframe and respond within a fraction of a second to whatever condition.

@JAB They're supposed to fly in circles or return to base when the connection is lost, but it's not unusual for them to crash instead.

@NickT - Your opinionated statements depend on a level of flight automation we have not yet achieved. For an O-1's pay grade, you can put a human in more or less direct control of an aircraft and get capabilities like image recognition, voice recognition, verbal command comprehension and independent decision-making that far exceed what $10 billion has given us with the Global Hawk (which is the only known program to have a command interface anywhere close to what you think all drones should have).

@KeithS true, I'm being a bit inflammatory, but at any pay grade and level of skill, a computer's understanding of aerodynamics and physics is superior. Not that I'm not advocating for total autonomy like you mention (image recognition, etc.), as in that realm computers are still fairly lacking.

Who says they are weird shaped? Maybe the 172 is weird...
The short answer is that it's the best design for the mission and the specific application. The 172 is designed to carry people and train young pilots so its characteristics reflect being a good fit of that mission. Drones are required to carry computers and surveillance equipment and often a weapons payload and very specifically, not people. Drones can be of a different shape than a GA plane since pilot ergonomics don't really matter. For what it's worth there are planes that are shaped like the drone you mention.
The Bonanza had a long run with the V tail design:

(source)
The new Cirrus Jet has a V tail and a bubble airframe much like the drones you pictured:

(source)
Bottom line is that planes are designed for specific applications and some airframe aspects lend themselves to a given mission better than others. Planes will reflect these constraints over looks 100% of the time.
Side note: low speed is by no means necessary for good recon. The SR-71, which was arguably one of the most successfully spy planes, did plenty of recon from 80000ft at Mach 3 with out any problems.

The SR-71 recon mission is just a little bit different from the drones' mission. I'd say low speed is an advantage when you want it to loiter over a target area.

Very true, i was just providing a counter example.

+1 for the fact that they don't have to carry people or deal with the weight where people normally sit.  The center of gravity can be adjusted as needed, and components can be moved to where they work best with no restrictions.

Another factor:  Cessnas, like virtually all manned aircraft are optimized to get to the destination.
Military drones, however, are more interested in dwell time over the area of interest than in how fast they get there.  This results in a very different optimization than for manned aircraft.
They also have a very different view of safety.
You have an aircraft that costs \$500k.  There's a safety measure that has a 5% chance of saving the airframe over it's expected life but which costs \$50k.  On a manned plane that's almost certainly going to be done.  On a drone it wouldn't make sense in most cases.

For single engine pushers with a canopy, when a pilot bails out of a manned aircraft they might hit the prop or intake at the rear of the aircraft. This requires either special provisions for ejecting, like falling out the bottom or side of the aircraft, or the installation of an ejection seat powerful enough to guarantee the pilot clears the propeller. This all adds weight and cost. A lot of pilots don't like this idea.
An unmanned drone has no such problem, aircraft designers are free to choose designs without considering how a human will get out of it.

A big part of the reason for the bubble shaped head is reduced drag. The area does hide a parabolic antenna but the could be done with many shapes. This shape is designed to have the smallest flat surface area possible to allow the air to flow smoothly across the front of the aircraft instead of becoming disrupted and turbulent by striking a flat surface. This leads to reduced drag and a longer flight time per gallon of fuel.
Cessna's and similar aircraft have a windscreen that is basically a large flat plate for the wind to slam against, and while they work great for seeing through, they actually add quite a bit of drag to the aircraft. The cessna's designed mission, however, makes this acceptable.

Welcome to Aviation. While the idea to decrease drag seems a good one, the spherical shape is not the best at it. More elongated profile (like wings) have proved to be more aerodynamic.

If you look at the front bottom side of a typical single engine Cessna you will find that it is black and oily from exhaust fumes and oil residues.

On reconnaissance UAVs, placing an engine at the nose (although there are a few examples - IAI Hunter for one) cause engine residues to accumulate on optical payload lenses during operation and degrade payload performance.
This is the main consideration for placing (piston) engines at the rear of reconnaissance UAVs.

Welcome to Av.SE!

So why not put the engine at the front with the exhaust pointing upwards? You can't do that on the Cessna because it would dump the exhaust residue onto the windshield, but that's not an issue on a drone.

high aspect ratios are provided for recon activities like lockheed martin U3 which had aspect ratio of 15 but this types of manned aircraft, pilot should be very high skilled

What types of aircraft? What does aspect ratio have to do with skilled pilots?


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Does load factor limit actually change with aircraft weight?
Does load factor limit in reality change with aircraft weight? Is it fixed to make certification and operation more simple? Now unloaded aircraft has lower Va speed. This makes sense to keep constant maximum G. But if aircraft can sustain +2.5g at MTOW will anything break if it sustains +3.0g completely empty? As I understand wing bending force will be the same and this should be limiting factor in most aircraft? What else can break? Engines fall off? Tail breaks off? Theoretically possible but can this really happen?

Yes, the limits are always set by the worst combination of weight, loading, flight speed and expected gust speed. They are valid over the full range of parameters for simplicity reasons.
Note that the load factor limits are not directly proportional to the aircraft's overall weight, but the weight of the non-lifting parts. An empty fuselage and full wing tanks allow the aircraft to pull more gs or fly faster until the maximum wing bending moment is reached when flying through the maximum expected gust. Calculating the precise load limit during each stage of a flight would put excessive demands on any pilot and increase the risk of errors.
All components should be sized so that they fail at 150% of calculated maximum loads. In real aircraft some secondary considerations like the availability of sheet metal gauges or extruded profiles put a limit to this local optimization, but technically the chance for breaking is the same for all structural parts. All the failures described in your question have happened in the past when aircraft were overstressed or improperly designed.

Not always. The T-38 G limits go up as aircraft weight goes down. This quiz site confirms my recollection: 3500# fuel, 5.7Gs, 3000# fuel, 5.9Gs, etc.

@RalphJ -- The T-38 has its fuel in the fuselage instead of the wings, which is why it does that.


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Any info on very strange wing endings on this Stemme glider?
Do you have any info on this strange wing endings used on Stemme glider?

Source

These wing endings were part of an experimental series aimed at reducing induced drag. These wing endings offer the performance of much longer wings.

@D_S If you have any kind of reference, I would upvote that answer.

"Courtesy of" means you got permission to repost the picture. Did you get permission or are you just saying where you found it?

This was a test aimed at measuring the effect of feathered wingtips. The Stemme S-10 has a four-part wing, and for the experiment the outer wing panels were removed and replaced by five staggered small wings, as shown on the picture in your question. This reduces wingspan from 23 m to only 11.4 m plus the span of the wingtips, in total maybe 14 m.
A feathered wingtip shows less induced drag than a straight wing of identical span, similar to a winglet, because it can involve a bigger mass of air into the creation of lift. But it cannot perform wonders, the effect is small.
The concept behind this experiment is called "winggrid" and the brainchild of Dr. U. La Roche. See here for one of the few still active sites on the topic - the original site http://www.winggrid.ch has long been abandoned.
Sad fact: This aircraft was destroyed in a mid-air collision in 2007. At that time, it had been reverted to the original wingtips.

So feathered wingtips were not very effective?

@Andrius: Don't expect miracles. They work well in special circumstances, and most birds are presumably very happy with them - only seafowl and swallows have pointed tips. As soon as you need to fly through the occasional thicket of branches, they are the preferred solution. Then of course without the tip reinforcement shown on the Stemme.

@Andrius: Now I found one site which still mentions the winggrid concept behind this experiment. I edited the answer accordingly. Don't click on the winggrid.ch link - this site is abandoned and now full of annoying ads.

@PeterKämpf, IIRC the main reason why most birds have feathered wing tips is that they are quieter, which is why they are so prominent in owls and birds of prey. Swallows and swifts that hunt in the air and so need high manoeuvrability and sea birds that can't hope to get help from thermals, and thus need efficiency most, have pointed wingtips.

@JanHudec: Ah, good point! My understanding so far was that feathered wingtips give more area at the tips and better maneuverability around twigs and branches, and all that with limited span. Seafowl have more span and raked wingtips, because the ocean surface has much less underwood.

@PeterKämpf, that likely also plays a role, especially for the birds that hunt in forests. It still doesn't show it would improve efficiency though. Most efficient are probably albatrosses who use dynamic soaring over sea between the boundary layer just above the waves and faster wind higher up and they have high aspect ratio wings with raked tips.

@JanHudec: Sparrows or pigeons don't hunt. Maneuverability is important for every bird on land. And spreading the feather tips vertically does help with efficiency (only a bit, but every bit counts).

@JanHudec - I know it's an old comment but-- when a Red-tailed hawk "stoops" on prey the wings are held back and the slots on the wingtips are basically closed-- so any quieting effect is lost.  Consider also the very prominent wingtip slots on Condors and Vultures-- which hunt rarely if ever.  Consider also the wingtip slots seen on soaring cranes like Sandhill Cranes and Whooping Cranes, which don't hunt from the air.  It's pretty clear that the slots are a good solution for birds that do a lot of thermal soaring, regardless of whether hunting or not.

The slots may also help a large heavy bird accelerate into flight when taking off.  The increase in "effective aspect ratio" comes into play here-- if a bird evolved to have a longer narrower wing with pointed tips and no slots (like the albatross), lifting off from the ground might in many situations be much more difficult than with the shorter, broader, lower-aspect-ratio wing with the "effective aspect ratio" boosted by slots.

The slotted wings are also optimized for rapid acceleration from a standstill-- I've read a description of how the extended slotted primary feathers bend in extreme way that acts like a propulsive spring when an accipiter hawk takes off from a perch to chase prey. On the other hand examples of birds with pointed wingtips w/ minimal slotting would be Ospreys, falcons (both of which also soar in thermals, but also are optimized for windy environments, and prolonged flapping flight (crossing water bodies during migration), swifts (which literally spend all day on the wing), and seabirds


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What does it mean when pitch numbers move all over the HUD screen?
In this video from an F-16:

You can see pitch angle move out of screen or be on the side at the end.

Most modern military aircraft HUDs have two primary methods of displaying the velocity vector (or flight path vector) relative to the pitch ladder.
Uncaged Mode
When the HUD is operating in uncaged mode, the velocity vector remains fixed between the pitch ladder and, instead, the pitch ladder slews around the HUD. It is important to note that even though the velocity vector remains fixed between the pitch ladder, the velocity vector will always point in the instantaneous direction the aircraft is headed (ie, in relation to the sum of all forces acting instantaneously upon the airframe). Thus, to compensate for the velocity vector's fixed position on the pitch ladder, the entire pitch ladder slews around the HUD to indicate instantaneous velocity. I've generally found this mode most useful for landing because the pitch ladder creates a massive target to bracket the field, and it is very obvious if there is any crosswind component because the pitch ladder will fly out as you roll onto final.
Caged Mode
In this mode the pitch ladder will remain in a fixed position and the velocity vector will fly out independently to indicate instantaneous velocity. Tactically speaking, this is generally the preferred mode because it is both less distracting (the giant pitch ladder isn't bouncing everywhere) and the pilot always has an artificial horizon to reference in the HUD during maneuvers. During tactical maneuvers its commonplace for the velocity vector to become HUD limited so having a stable horizon to reference greatly increases situational awareness.
This Video
To your specific question, this F16 is operating the HUD in uncaged mode. Every HUD has visual limits (JHMCS withstanding) and there are obvious visual limits on the HUD in this video. As the pilot commands the jet to increase the G-Load you can see that the pitch ladder rapidly disappears to the right of the screen. This is indicating that the aircraft is in a hard left hand turn and that the pitch ladder has become HUD limited. Once the G-loads relax, you can see the pitch ladder fly back into the center of the HUD. The pilot finishes the maneuver inverted by overbanking through the left hand turn.
Further Comments
Upon further review of the video it appears the pilot either ALOCs or GLOCs partway through his break turn at just over 8 Gs. This video is a pretty terrifying look at the realities of BFM training in the military. They started their exercise at 16,000ft and the pilot aircraft's Auto-GCAS in the video makes a supersonic recovery at 660kts, slightly below 3000ft AGL! At that speed ejection could lead to possible death or permanent disfigurement, and the ground is a mere seconds from impact. His recovery altitude is well below safety altitude for BFM, and arguably within immediate ejection considerations (There was an eagle driver that punched out at Mach 1+ during a similar GLOC episode when he woke up pointed at the ground seconds from death. His ejection nearly killed him.)
The pilot Auto-GCAS begins recovery at 55 degrees nose low, 8,500ft MSL at 665 kts. The recovery is a max performance 9 G pull and takes roughly 7 seconds, during which point, while recovering, the aircraft loses an additional 4,500ft. Just think, if the aircraft lost 4,500ft during a 9G recovery, impact with the earth was well under 10 seconds had he not regained control of the aircraft.

Also, is the description in the other answer of boresight pitch minus FPV pitch = AOA correct, or is there another factor missing in that explanation?  Thanks!

So at the end of the video if I understand correctly the nose points 10 degrees up and FPV is 15 degrees up? Speed is stable. How can be this possible? In normal flight FPV usually points several degrees below nose? Where is g displayed? At the beginning heading is also not shown. Do you know why?

@RalphJ Gravity-induced Loss of Consciousness and Almost Loss of Consciousness, or so Google says.

@RalphJ Yes. The velocity vector represents instantaneous relative motion through the current body of air, and the nose of the aircraft is often in line with the chord of the wings, so, perhaps with some academic differences, the angle between them is the current AoA.

@Andrius Your question about the FPV would make a great question all on its own. As for the G, its indicated on the left above the airspeed indicator. I've never seen an F16 HUD, but I imagine they all operate pretty similarly. The heading is not initially shown because either the pilot has selected a specific Air-to-Air mode which either automatically declutters the HUD and removes non-related data, or he manually decluttered it. You can see he switches modes just prior to starting his break turn for the next set and the heading returns to the HUD.

It was explicitly stated on the video itself (and followed up in more detail in the article linked in the description) that the pilot did lose consciousness and that the plane recovered itself using a built in safety system.

@DanNeely pretty interesting read, thanks for the heads up. I was almost tempted to say he was a student--the article confirms it. I added the corrections in my answer. Pretty interesting stuff, I'm not so sure I would trust an Auto-GCAS system like this particular Viper has. Specifically in the F18, Betty has tried to kill me a few times when she got confused and told me to roll directly into the terrain.

Glad to see you're back around, RhinoDriver! Been wondering where you've been. Always like reading your answers


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Why do none of the procedures for EFSA use the SVL VOR-DME?
This is inspired by this question. Looking at all the approach and STAR charts for EFSA in Savonlinna, Finland, they are all RNAV. The Linna VOR-DME is not used for any of them even though it is located a half mile off the arrival end of runway 12. Why is this navaid not used for any of the procedures?

According to the Finnish AIP, LINNA (SVL) is only a DME, not a VOR/DME.

I think it used to be a VOR/DME, but the VOR was probably discontinued for economic reasons.

Yep, that is the general plan for many VOR/DME in Europe, I think. Presumably it would be a good idea to revert that trend now, at least in areas affected by Russian intentional or "collateral" GPS jamming.

@tml Most (all?) airliners can still do RNAV from DME/DME/IRU in the event of a GPS outage. No VORs needed.

@StephenS Would the DME distances need to be listed on the charts?

@TomMcW Nope. The FMS already knows where all the DMEs are, so it can triangulate its own lat/long. It’s not as accurate as GNSS, of course, but RNAV/RNP was designed with different levels specifically to account for that.

@StephenS Sure, DME/DME is enough for enroute RNAV, but the approaches at EFSA are RNP or ILS with GNSS required, so not enough. When SVL was still a VOR/DME, they probably had a VOR approach or a VOR-based procedure turn onto the ILS.

@Bianfable I don’t see any good reason to require GNSS on that ILS, and you wouldn’t even need RNAV at all if there were a teardrop or PT off the VOR as is common in many other countries. Perhaps Finland just doesn’t want you flying there if you don’t have all the latest toys.

The VOR was discontinued a few years back, they've been disabling VORs one by one here in Finland. It's only a DME now. (I know, I've flown a Cessna to Savonlinna a few times.)


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What bank angles are used for IFR operations?
ICAO requires or recommends 25 degrees bank angle. On Boeing auto pilot it's possible to set 30 degrees to be used. What bank angles usually are used for Holding, ILS intercept en-route when flying manually and on autopilot? Any EASA rules in this field?


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Will EASA approve single engine IFR commercial operations any time soon? Will this be a big impact on the industry?
I saw that discussions were going on for EASA to approve single engine turbine airplanes for IFR commercial operations. Anybody knows the current status? Where can I find it? Is it going ever to happen? Another question is how big impact this is going to make? Are SAT much more efficient and cheaper to run than multi engine aircraft?

What discussion?

By commercial do you mean passenger service? At least in the US (not sure about EU) there are quite a few single engine turbine planes approved for IFR commercial ops, typically for cargo missions (FedEx actually has quite a few).

Yes, passenger service


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On the A330 why does switching off the ADRs require alternate LG extension and inhibit nose wheel steering?
In the incident examined in this video the pilots experienced an unreliable airspeed indication due to pitot tube blockage in an A330. They followed the abnormal checklist for unreliable airspeed which instructed them to switch off all three ADRs.
In the video it is mentioned that with the ADRs switched off that they had to use gravity extension of the landing gear, which caused one of the gear doors to briefly strike the runway. Also it inhibits nose wheel steering requiring a tug to get the plane off the runway.
I can’t figure out why the lack of ADRs would affect these two systems. I know that gear extension would be inhibited above a certain airspeed, but it seems that switching off the ADRs should override this. Why would it require alternate gear extension? And why would nose wheel steering be affected on the ground?

And why would gravity extension cause the gear door to strike the runway??

@Jim Gravity extension leaves the doors hanging open when they would normally close after extension. They were landing overweight so they rubbed the runway.

Ahh. Ok, I see.


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Are there any low time lucky CPL holders who operate as PIC on medium or heavy jets?
CPL holder can be PIC on any plane if it's not a commercial air transport. So if the flight is positioning, acceptance, testing, delivery or any other without cargo and passengers any CPL holder provided he/she has TR and special training for the left seat can act as PIC? Do airlines allow this maybe as bonus for young first officers?
Another option is big business jets like 737 747 BBJ, A319...

I doubt it because it would also require a type rating. The issue is that to be PIC in a type rated aircraft requires quite a few hours.

It is possible but hard: http://aviation.stackexchange.com/questions/23404/can-a-private-pilot-fly-commercial-type-aircraft

Being PIC on a multi-crew aircraft requires an ATP(L) in, I think, every jurisdiction. Which comes after 1500 hours.

Which country's regulations are you asking about?  (Please add a tag such as faa-regulations, easa-regulations, etc. so that we know how to answer.)

There have been periods of time in the past (and I think at present as well) where a lack of experienced pilots meant airlines had to resort to hiring CPL's as SIC's on turboprop and some regional jets. I know that American Eagle did that for a while -- they used to call them the 500 hour wonders. Some were hired with even less time if they showed exceptional skills.
However, to log PIC time in an aircraft that requires an ATP ticket, you have to have the ticket. Sole manipulator of the controls is not enough for a CPL to log the time as PIC on those aircraft.

In what regulatory environment is your answer valid?

USA FARs. Here's some FAA guidance on the subject. link

Aircraft don't require an ATP in the US, but certain operations do (121, 135 with passengers, etc.)

For that matter, your entire second paragraph is incorrect.  Even in operations requiring an ATP in order to be PIC, if the SIC is rated in the aircraft they can log PIC for that time where they act as sole manipulator of the flight controls regardless of the fact that they are not legally able to be the PIC.  (This is consistent with the link that you posted in the comment as well.)


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Why afterburner fuel nozzles aren't creating a lot of drag during normal operation?
I cannot understand how afterburner fuel nozzles are not crating a lot of backpressure and allow engine to work when afterburner is not in use? They close almost 50% of the exhaust area. Is there any efficiency loss during normal operation? Can nozzles move out of the stream when afterburner is not in use?

The gas in the afterburner tube is relatively slow. Pressure isn't converted to velocity until the very end of the nozzle.

"allow engine to work when afterburner is not in use" because the engine is specifically designed to work with them there. You can't just add an afterburner to any old engine and call it a day...

I don't believe that those are the fuel nozzles (more correctly called the fuel spray bars). Here's a schematic of a turbojet with and without afterburner.

image from aerospaceweb.org
The fuel spray bars are very thin. The afterburner flame holder is much thicker, because its purpose is maintain combustion by slowing the air and mixing the fuel and air so complete combustion can occur within the engine.
I believe the object in the picture is the flame holder, which needs to slow airflow to do its job. Counterintuitively, the engine wouldn't make as much power without the flame holder because the flame would blow out of the engine before the air and fuel finished mixing and burning.
I am surprised that it doesn't interfere with airflow when the afterburner is not in use. However, notice that in the diagram (and suggested in your picture) the engine gets wider. It doesn't take a big increase in diameter to double the area of the engine. That means that the open space around the flame holder is not that much smaller than the unobstructed width at the spray bars. I can't find a source to prove that explanation is correct though.
You can see these parts in this YouTube video: Afterburner Fuel System

Is there any efficiency loss during normal operation?

Yes, indeed a couple of percentage points of dry thrust is lost due to the afterburner assembly.
But everything visible in that picture is needed and is needed in that position,  mainly for one simple reason: the primary limiting factor of a combustion (everywhere it happens, either in a combustion chamber or in an afterburner) is its speed. Combustion is an extremely slow process, so slow that burning velocity is normally measured in cm/s (inches/s). The main components of an afterburner are therefore designed taking into account this limitation. Starting from the low pressure turbine and moving downstream, we can find:

a diffuser (i.e. an area-increasing section); this is the easiest way to slow down the flow which has normally a value of a couple of hundreds of m/s just after the turbine;
a row of simple annular rings with holes; these are the fuel injectors supplying fuel downstream; several are needed with different radius and in different positions in order to have a smooth combustion; due to the still relative high airflow speed, fuel is broken apart as soon as it exists the holes, vaporises and mixes with the air-fuel mixture coming out of the turbine; this mixture has still a lot of air in it since the fuel in the combustion chamber upstream is burned in excess of air to limit temperature on the turbine's blades; so, air is here, atomised fuel is here, temperature is high enough, there is nothing left to do but further reducing the speed down till the burning velocity, as said before; this is achieved by a;
flame holder; this is a simple row of v-shaped bodies which create a lot of recirculation behind them and stabilise the flame i.e. give time to the flame to develop and burn.

The following cutaway shows a Pratt & Whitney F100:

I have highlighted in blue the diffuser, in green two of the seven rings of fuel injectors and in purple one of the v-shaped flame holders.


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How to guess the actual name of the SID, waypoint, etc., if you have never seen it when told by ATC?
Sometimes it's very tricky as those identifiers are not normal words. For example how to find a way-point you have to go direct to if it's not on the flight plan? SID and STARS can be reviewed before clearance but some airports have a very big list.
Can anybody give any good advice on this? How do you deal with this situation?

You can't, there is no system to them (at least no globally consistent system, airports may have a system for their own STARs and SIDs, and some countries for numbering airways, but nothing worldwide).
What is common (but afaik no law/treaty) is for STARs and SIDs to be named after the entry/exit waypoint, which can help predict which you're likely to get based on the direction you're departing or arriving.
E.g. at Amsterdam all the LEKKO departures end at waypoint LEKKO, all ARNEM departures end at ARNEM (which just happens to be situated over the city of Arnhem, but there's no international standard there either).
Some waypoints or navaids (see above) may in some more or less obscure way refer to their location as well.
For example Chattanooga, TN has a VOR called Choo Choo.

It is common, but I heard it is now discouraged for new waypoints by EUROCONTROL. As far as I know no new waypoints in recent years created in our country are named that way although the old ones were.

authors: Vladimir F Героям слава


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What type of airspeed are wind speeds given in?
When you get wind speed from an ASOS/AWOS or from ATC, are those speeds in true airspeed? Are the instruments weighted for elevation?
Say you're on approach to 05R at MEX and you're given wind speed of 25 kt at 140° and the OAT is 35°C. Do you need to take density altitude into account to determine if it's safe to land? Or is there not enough of a difference to matter?

The "wind speed" is not an instantaneous value. It's the mean value of 24 measures which are themselves a mean value over 5 seconds (total is 2 minutes). The measures are done at 10 m AGL, but if they were done at 5 m, the values would be 13% smaller due to viscosity. Also the wind vertical component is not taken into account. This gives an idea of the accuracy, still I wonder how are anemometers calibrated. Good question.

I think some of you confusion stems from learning about calibrated, indicated, and true airspeed of an airplane. Indicated and true airspeed vary with elevation because the number of air molecules hitting the pitot tube at high altitudes is less than at lower altitudes. That’s why there is a correction factor for altitude. As explained in @ymb1's answer you don’t need to do that when measuring wind speed.

@JScarry: Still Tom is right when he says the effects of the wind depend on air density (contrary to what is said in the answer). However for a cup anemometer, air acts both on the concave retracting cup than on the two other advancing cups (which show their profile or their convex side) and I suspect the opposed forces cancel out most of the density variation effects. I'm not sure this would be the case on a propeller-type anemometer.

Surface winds are given in magnetic headings.
In contrast, winds aloft forecasts are given in true headings.  They are not adjusted for DENALT.
Indicated airspeed (IAS) is the measurement, subject to instrument error. Indicated airspeed is really just a dynamic pressure reading.
Calibrated airspeed (CAS) compensates for instrument errors.
True airspeed (TAS) is CAS adjusted for air density (pressure and temperature). TAS does not take into account winds, and therefore is not a true ground speed.
Surface winds are given in a ground relative flow of air, and are relative to a magnetic heading.
Winds aloft are forecast and reported in an airspeed, and are intrinsically based upon the ground relative flow of air, and are relative to a true heading.
Therefore the AWOS/ASOS/tower reported surface winds are not in TAS, as that term is defined as being a vehicle vector magnitude.
However, when at Leadville, CO, (KLXV) the measurements of the airflow are relative to a datum slightly above the ground, and in that sense are analogous to the true airspeed of an aircraft if it were flying stationary above the airport.  But TAS is not the correct term. It is simply a surface wind velocity and the corresponding magnetic directional heading.

I didn’t figure that TAS was the correct term, but I couldn’t think of any way to express it. But you get the point, is it analogous to TAS or is it affected by altitude. I used a conversion calculator on the web and found that at KLXV elevation, 30kt TAS converts to 25.8kt EAS, so a 14% difference. Not sure how much difference that would make in the scheme of things. It will effect the flight characteristics of the plane by 25.8kt instead of 30. A hot day might make even more of a difference.

The crosswind of 20k at KLXV will have the same effect on your plane as a crosswind of 20k at KMIA.

What about head and tailwind? I'm thinking, if you're sitting on the runway at sea level facing a 30kt headwind your IAS shoulda read 30kt. But if you're at KLXV that same headwind would read 25.8. So you'd need a slightly longer takeoff roll to reach Vr. Or am I thinking wrong?

30 kt at KLXV in  @TomMcW question


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why there is APU bleed air supply?
Does someone know anything about APU bleed air supply in this pic?

The APU is an alternate source of electrical power and also bleed air.  Since it doesn't have any function to generate thrust, it burns considerably less fuel than the engines do, and is intended primarily for ground use, although most APU's can also be operated in flight for things like a second source of electrical power if an engine generator has failed, or a source of bleed air if the bleeds are closed on the engines in order to generate more thrust.
In that diagram, what you're seeing is a portion of the pneumatic system, showing that the bleed air from the APU can be routed to both air conditioning packs (on many aircraft the APU can power both AC packs on the ground; on some aircraft it can only power one at a time), as well as to the engines, where it provides air for starting them.
On some aircraft, the bleed air from the APU can do other things, like pressurizing the water supply, and while it isn't operationally useful, it can generally supply bleed air into the anti-ice ducting for the leading edges.  In practice, though, that's only done when the engines are running, due to the greater volume (and temperature) of hot bleed air that the engines can produce compared to the APU.

APU is there for the bleed (and running some electrics).
Using that bleed an aircraft at the gate (with engines shut off for ground personnel safety reasons) can run the airconditioning and start the engines without needing pressurized air supplied from the ground from a huffer cart.


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How accurate is an Inertial Navigation System
I was surprised to learn that an inertial navigation system even works at all; I'd expect that the integration drift would throw the velocity measurement off badly enough, not to mention the position. Other questions have asked about how these systems work, and the answers have explained that cross-checks with other measurements which give absolute position (e.g. GPS) improve accuracy.
But given a state-of-the art INS which operates in isolation (no interfacing with airspeed, GPS, etc) from takeoff, how much drift could we expect for, say, a normal transatlantic flight? Does this change if the aircraft flies through turbulence?